This invention relates to pyroelectric detector arrays for sensing electromagnetic radiation.
Radiation detectors can generally be divided into two broad categories, according to the nature of the energy detected. Quantum or photon detectors respond to discrete excitations caused by the action of individual photons. Thermal detectors are sensitive to changes in the temperature of the detector material caused by the absorption of energy from the incoming radiation. This absorption of the radiant energy may be direct, by adsorption within the detector material itself, or indirect through absorption in some auxiliary structure which conducts the heat to the detector material.
Among the various types of thermal detectors, pyroelectric detectors utilize the spontaneous electrical polarization of a pyroelectric material, which results from the anharmonic ionic vibrations possible in these classes of crystals which lack a center of symmetry. When the temperature of a pyroelectric detector is changed, the temperature change alters the spontaneous polarization of the material which, in turn, causes a charge to flow. The charge can then be measured and related to the intensity of the incoming radiation.
Pyroelectric detectors may be incorporated into a focal plane array, where the scene to be viewed is optically focussed on a two dimensional matrix of detector elements so that each detector images a particular portion of the scene. The use of focal planes has become particularly desirable in the field of infrared imaging with the advent of improved signal processing techniques and photolithographic processes which allow the fabrication of high density infrared systems employing a large number of detectors per unit area.
Although focal plane research has in the past concentrated on photovoltaic detector designs, the need for cryogenic cooling for such detectors has led to the consideration of thermal detectors for use in medium performance applications. Thermal detectors do not require cooling and, as a consequence, are inherently simpler in design than photovoltaic detectors. In addition, thermal detectors are uniformly sensitive over a wide range of the infrared spectrum and exhibit a nearly constant signal to noise ratio over a large frequency range.
Ideally, a pyroelectric detector would be totally isolated from its surroundings so that thermal losses from the detector would occur only by radiation. In practice, however, some sort of mounting to a substrate is required, and heat diffusion into the substrate degrades the responsivity of the detector. Thus, one desirable feature of a pyroelectric detector design is to provide a detector mounting scheme which minimizes heat transfer between the detector and the mounting structure.
Another goal in pyroelectric detector design is to minimize the mass of the pyroelectric material, because a smaller thermal mass will change temperature in response to absorbed radiation more quickly and with a higher responsivity than a larger mass, leading to a faster and more sensitive detector.
Minimizing the detector mass usually involves thinning the detector material, but this approach conflicts with the need for maximum thermal isolation, since a minimal amount of support is desirable for reduced heat transfer, but thinner layers of pyroelectric material are more fragile and therefore require more substantial support structures. Consequently, the introduction of a pyroelectric detector array design exhibiting increased thermal isolation for a relatively thin detector without sacrificing the physical integrity of the detector structure would be an important contribution to the art in the thermal imaging field.